An advantage for some security applications would be to keep a unique chip identification value a secret. If the chip identification value could be read from outside the packaged chip, then the secret would be exposed to hackers. If the chip identification value were kept confined inside the packaged chip, and used only by internal chip resources, then efforts by hackers to discover the secret are greatly frustrated. Furthermore, if each chip contains a statistically-unique identification value, no global secret (i.e., shared among devices of the same category) would exist to be discovered. The absence of a global secret greatly reduces a payoff of any hacking effort and thus should reduce an amount of effort a hacker would be willing to invest in mounting an attack.
Problems with using chip identification values for cryptography are cost or repeatability. On-chip programmable read only memory cells, nonvolatile memory cells, fusible links and laser trimmed circuits use special fabrication processing and/or extra programming steps to establish the identification value. Random identification values established during fabrication produce measured values that are not perfectly repeatable. See U.S. Pat. No. 6,161,213 issued to Lofstrom where variations between MOSFET pairs are measured to generate a “silicon” identification value. Due to measurement fluctuations when the two MOSFETs both have similar channel cutoff voltages, a bit error rate (BER) of about 1 to 2 percent can arise between successive readouts. A varying silicon identification value used as a cryptographic key value cannot tolerate random changes (i.e., a decryption key measured at a particular time would not correspond to an encryption key measured at another time).